Abstract
Abstract. Air quality networks in cities can be costly and inconsistent and typically monitor a few pollutants. Space-based instruments provide global coverage spanning more than a decade to determine trends in air quality, augmenting surface networks. Here we target cities in the UK (London and Birmingham) and India (Delhi and Kanpur) and use observations of nitrogen dioxide (NO2) from the Ozone Monitoring Instrument (OMI), ammonia (NH3) from the Infrared Atmospheric Sounding Interferometer (IASI), formaldehyde (HCHO) from OMI as a proxy for non-methane volatile organic compounds (NMVOCs), and aerosol optical depth (AOD) from the Moderate Resolution Imaging Spectroradiometer (MODIS) for PM2.5. We assess the skill of these products at reproducing monthly variability in surface concentrations of air pollutants where available. We find temporal consistency between column and surface NO2 in cities in the UK and India (R = 0.5–0.7) and NH3 at two of three rural sites in the UK (R = 0.5–0.7) but not between AOD and surface PM2.5 (R < 0.4). MODIS AOD is consistent with AERONET at sites in the UK and India (R ≥ 0.8) and reproduces a significant decline in surface PM2.5 in London (2.7 % a−1) and Birmingham (3.7 % a−1) since 2009. We derive long-term trends in the four cities for 2005–2018 from OMI and MODIS and for 2008–2018 from IASI. Trends of all pollutants are positive in Delhi, suggesting no air quality improvements there, despite the roll-out of controls on industrial and transport sectors. Kanpur, identified by the WHO as the most polluted city in the world in 2018, experiences a significant and substantial (3.1 % a−1) increase in PM2.5. The decline of NO2, NH3, and PM2.5 in London and Birmingham is likely due in large part to emissions controls on vehicles. Trends are significant only for NO2 and PM2.5. Reactive NMVOCs decline in Birmingham, but the trend is not significant. There is a recent (2012–2018) steep (> 9 % a−1) increase in reactive NMVOCs in London. The cause for this rapid increase is uncertain but may reflect the increased contribution of oxygenated volatile organic compounds (VOCs) from household products, the food and beverage industry, and domestic wood burning, with implications for the formation of ozone in a VOC-limited city.
Highlights
More than 55 % of people live in urban areas, and this is projected to increase to 68 % by 2050 (UN, 2019)
We assess the ability to use the long record of satellite observations of atmospheric composition to monitor long-term trends in surface air quality in cities in the UK (London, Birmingham) and India (Delhi, Kanpur) of variable size, at a range of development stages, and with air pollutant concentrations that pose a greater risk to health than previously thought (Vodonos et al, 2018; Vohra et al, 2021)
We assessed the ability of satellite observations of nitrogen dioxide (NO2) and formaldehyde (HCHO) from Ozone Monitoring Instrument (OMI) for 2005–2018, ammonia (NH3) from Infrared Atmospheric Sounding Interferometer (IASI) for 2008–2018, and aerosol optical depth (AOD) from Moderate Resolution Imaging Spectroradiometer (MODIS) for 2005–2018 to provide constraints on long-term changes in city-average NO2, reactive non-methane volatile organic compounds (NMVOCs), NH3, and PM2.5, respectively in four cities: two in the UK (London and Birmingham) and two in India (Delhi and Kanpur)
Summary
More than 55 % of people live in urban areas, and this is projected to increase to 68 % by 2050 (UN, 2019). Satellite observations of atmospheric composition (Earth observations) provide consistent, long records (> 10 years) and global coverage of multiple air pollutants, complementing surface monitoring networks with limited spatial coverage and temporal records (Streets et al, 2013; Duncan et al, 2014). These have been used extensively as constraints on temporal changes in surface concentrations of air pollutants and precursor emissions (Kim et al, 2006; Lamsal et al, 2011; Zhu et al, 2014) but typically just targeting one–two pollutants. We conduct a systematic evaluation of the ability of satellite observations of NO2, NH3, HCHO, and AOD to reproduce the temporal variability of surface air pollution in the UK and India before going on to apply these satellite observations to estimate long-term changes in air pollution to assess the efficacy of air quality policies in the four cities of interest
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